1. Field of the Invention
The invention relates to a process for improving the gravure and offset printability of coated and uncoated printing papers by increasing the filler share in these papers on the top and bottom sides or in the outer layers of the paper web by the use of retention agents.
2. Discussion of the Prior Art
According to the prior art, so-called SC and LWC rotary printing papers, which must meet higher standards for printing results, are used for large print jobs. The term "SC papers" refers to rotary printing papers that contain up to 35% mineral filler and have high gloss and smoothness due to post-treatment in a super-calendar. The abbreviation "SC" stands for "super-calendared."
The term "LWC papers" refers to rotary printing papers that contain up to 15% mineral filler by mass and are provided inside or outside the paper-making machine with a pigmented surface coating, the coat, on both sides. After being coated, these printing papers are also treated with a super-calendar to attain high gloss and smoothness. The abbreviation LWC stands for "light-weight coated."
SC and LWC printing papers were initially produced on so-called endless-wire paper-making machines. These papers have the disadvantage known as "two-sidedness," i.e., due to one-sided dewatering on the wire, filler and fine material were washed out on the bottom side, so that the wire side of the paper was always rougher than the top side, and the paper web, seen in the Z-direction, had an irregular or skewed ash cross-section. Varying filler or ash contents inevitably lead to irregular printing results on the top and wire sides, so that such papers no longer satisfy modern requirements.
To avoid such disadvantages, wire devices with two-sided dewatering, known as hybrid formers, were therefore developed.
The German publication "Paper Fabrication Weekly" ["Wochenblatt fur Papierfabrikation"], 1987, pages 461 through 478, provides an overview of modern sheet formation systems, consisting of flow box and sheet formation devices. Section 3, of this publication in particular, describes hybrid and twin-wire formers and their advantages compared with the classic endless-wire models.
In addition, the "Paper Fabrication Weekly" ["Wochenblatt fur Papierfabrikation"], 1987, pages 662 through 667, describes multi-layer printing papers formed by means of multi-layer flow box devices and twin-wire formers. The same reference also discloses that it is possible to deliberately influence the distribution of filler in the outer layers of the paper web by adding filler to the outer channels of a multi-layer flow box.
Finally, German reference DE 42 37 309 discloses a process for producing a fibrous web, wherein a fiber-bearing liquid is evenly distributed, with the help of a flow box, on a wire or between two wires. To avoid fluctuations in the area weight cross-section, the retention of fiber-bearing liquid on the wire is sectionally influenced by the sectional dosing of retention agent in the flow box.
Modern twin-wire formers for producing SC and LWC papers are operated at working speeds of up to 1,500 m/min. Known SC papers produced with these devices have filler shares of up to 34% with very even distribution in the Z-direction. These SC printing papers currently constitute the highest paper class with respect to printability.
Continued increases in production speed, including in twin-wire formers, are urgently needed to increase profitability. However, this raises the problem of intensifying the dewatering of the fiber-bearing liquid between the two wires along a given distance. This is done by applying a higher vacuum to individual dewatering elements, which engage with the upper and lower wires. However, the increased water extraction and the higher flow speed that accompanies it inevitably create the disadvantage that more filler is applied in the Z-direction of the formed wet paper web. This results in filler impoverishment on the outer sides, and thus in a substantial deterioration in printability.
German reference DE 37 20 618 A1 attempts to offer an improvement by providing a process to improve the filler retention. The fiber-bearing liquid is supplied to a multi-layer flow box after first being divided by fractionation into a long-fiber portion and a short-fiber portion, which also contains all fillers. The long-fiber portion is subjected to additional fibrillation by beating and, after dilution and the simultaneous addition of retention agent, is supplied to the outer channels of a multi-layer flow box in front of, the pulp pump by means of the pulp pump. The portion containing short fibers and filler is supplied to the middle or inner channels of the multi-layer flow box. Upon subsequent dewatering, the long fibrillated fibers of a fine network are formed between the two wires of a twin-wire former. The fine network, as a filter layer, serves to retain the filler and fine material of the middle layer and results in an increase in filler.
It is disadvantageous in this process that the anchoring of the outer layer containing the long fibers is impaired by the increased filler and fine material content of the middle layer, so that the Z-strength is decreased. The paper tends to split when stressed in the Z-direction, e.g., during offset printing.